Mass spectrometry is an analytical method for quantitatively and qualitatively determining the chemical composition and molecular structure of sample materials. Mass spectrometers are generally comprised of an ion source, a mass analyzer, and a detector. In operation, the sample is positioned in an evacuated area containing the ion source and an ion beam comprised of primary ions is directed at the sample surface. The primary ions collide with the surface species of the sample in accordance with classical collision kinematics, resulting in the back scattering of the primary ions and/or the ejection of surface species from the sample surface. Depending on the angle of incidence, mass of the primary ion beam, and energy of the primary ion beam, the ejected surface species may be comprised of elemental ions, neutral atoms, and/or molecular fragments. The back scattered primary ions and/or ejected species are focused and separated in the mass analyzer and detected by the detector. The energies (velocities) of the back scattered primary ions and ejected surface species correlate with the mass of the surface species and thus are used to identify the chemical composition and structure of the sample surface.
One type of mass analyzer is a linear time-of-flight (ToF) mass analyzer which determines the mass spectra of the surface species by measuring the times for the back scattered primary ions and the ejected surface species to traverse a field-free drift region. The field-free drift region is generally bounded by a drawout grid and an exit grid, which are often at ground potential. The primary back scattered ions and ejected species pass through the drift region and their times of flight are measured by the detector. Mass separation occurs because ions with different masses reach the detector at different times. Pulsing the ion beam, as opposed to directing a continuous beam of ions to the sample surface, allows for a discrete measurement of the back scattered primary ions and the ejected surface species at the detector.
As the primary back scattered ions and ejected species have different initial kinetic energies upon leaving the sample surface, a reflectron is typically used in conjunction with the ToF mass analyzer. The reflectron compensates for the initial kinetic energy distributions by providing a retarding electrical field that reverses the trajectories of the traveling primary ions and ejected species to negate the effects of the uneven kinetic energy distribution and differing velocities. As the ions enter the reflectron, ions with higher kinetic energy and velocity penetrate farther into the reflectron than those ions with lower kinetic energy and velocity, thus traveling a longer path to their focal point. In this way, primary ions and ejected species having the same mass but different initial kinetic energies arrive at the detector simultaneously. The detector counts the incidence of the ejected species. Thus, ToF analyzers including reflectrons can provide a mass spectrum for ejected species over an entire mass range with improved mass resolution verses a linear ToF analyzer.
Ion Scattering Spectroscopy (ISS) is a mass spectroscopy method that measures only the energies of the back scattered primary ions. The primary ion beam strikes the sample surface at about normal incidence, and the back scattered primary ions lose energy according to classical two-body collision kinematics. The surface species are identified by their mass, which is calculated from the arrival time (kinetic energy and velocity) of the back scattered primary ions. The back scattered ion signal is believed to be representative of the composition of the uppermost atomic layer of the sample. Using ISS, all elements heavier than the primary beam can be detected.
Secondary Ion Mass Spectroscopy (SIMS) is a mass spectroscopy method that detects surface species ejected by multiple collisions, also referred to as multiply recoiled or indirect ions, initiated by the incidence of the primary ions from the ion beam on the sample surface. FIG. 1 schematically illustrates SIMS, where the incident primary beam induces a collision cascade in the surface region, which dissipates energy to the lattice atoms through a number of successive biparticle collisions. As some of the cascade returns to the surface, molecular fragments and elemental species are ejected. The ejected surface species have low kinetic energies of less than 20 eV.
Direct Recoil Spectroscopy (DRS), as shown schematically in FIG. 2, is a mass spectroscopy method for measuring the kinetic energies of direct recoil surface species, which are surface species ejected by a single binary collision between a primary ion of the ion beam and a surface atom. DRS directs the primary beam at the sample surface at an angle (grazing incidence), such that binary collisions between the primary ions and the surface species occur, resulting in the direct ejection of surface species in a forward scattering direction, rather than in a collision cascade within the surface region. The energy of the DRS collision causes complete molecular decomposition, and only elemental species (ions and neutrals) are ejected and detected. In contrast to SIMS, the energy of the DRS ejected species is high (200 eV to 6 keV), depending on the scattering geometry, the recoiled mass, the primary ion mass, and the primary ion energy. Mass Spectroscopy of Recoiled Ions (MSRI) is a DRS method that does not measure neutrals, but only the elemental ions, resulting in a higher resolution energy peak for the detected elements.
The method and geometry of ion beam surface analysis (ISS, SIMS, DRS, and MSRI), as shown in FIG. 3, generally consists of directing an ion beam of mass M.sub.1 and kinetic energy E.sub.0 at the surface of the sample, which is comprised of atoms with mass M.sub.2, and detecting the back scattered primary ions with energy E.sub.1 (ISS), multiply recoiled surface species with energy of about 20 eV (SIMS), and/or direct recoil surface species (DRS/MSRI) with energy E.sub.2. For primary ions in the approximate range of between 1 keV and 100 keV, the primary ion-target atom collisions are adequately described by two-body classical collision dynamics. The kinetic energy E.sub.1 of the scattered primary ions is given by EQU E.sub.1 =(1+a).sup.-2 [cos q.sub.1 .+-.(a.sup.2 -sin.sup.2 q.sub.1).sup.1/2 ].sup.2
provided M.sub.2 &gt;M.sub.1. The kinetic energy E.sub.2 of the recoil surface species is EQU E.sub.2 =4a(1+a).sup.-2 cos.sup.2 .theta.
where a=M.sub.2 /M.sub.1 and q.sub.1 and .theta. are the scattering and recoil angles, respectively. As the mass and the velocity of the primary ions of the ion beam are known, and the velocity of the back scattered primary ions and/or ejected species is measurable, the mass of the back scattered primary ions and/or ejected species is determinable from the relationship E=1/2 mv.sup.2.
ToF SIMS instruments measure the times for the primary ions and low energy surface species ejected by the collision cascade to travel through the field-free region. The reflectron analyzer used in high resolution ToF SIMS instruments is positioned with the horizontal axis of the field free region close to the sample surface normal, such that the low energy SIMS ions are ejected into the analyzer. Advantageously, SIMS instruments detect and measure molecular ions and molecular fragments, as well as elemental species, providing valuable qualitative analysis of the chemical composition of the surface. Analysis of the mass data is complicated, however, when molecular species have the same mass as elemental ions (isobaric interferences). For example, C.sub.x H.sub.y molecular fragments prevent the positive identification of N (vs. CH.sub.2), O (vs. CH.sub.4), Al (vs. C.sub.2 H.sub.3), Cr (vs. C.sub.4 H.sub.4), and Fe (vs. C.sub.4 H.sub.8), and, more significantly , especially for the semi-conductor industry, the presence of CO and Si are indistinguishable, as well as Fe.sup.2+ and Si. Charge transfer and neutralization further complicates SIMS analysis. During the ejection of ions from the surface of the sample, a transfer of charge occurs between the surface and the ions, resulting in the neutralization of a portion of the ionic species. The probability of neutralization depends on the local electron density of the surface in the region from which the ion originated and the velocity of the ion as it exits the surface. In SIMS, ions are ejected from the surface with low velocities and kinetic energies, and the probability of ion survival varies by many orders of magnitude, depending on the element being ejected and the oxidation state of the surface. Thus, SIMS instruments measure a small fraction (less than 1%) of a large number surface atoms.
ToF MSRI instruments measure the times for the primary ions and high energy surface species ejected by a single binary collision to travel through the field-free region. MSRI instruments do not measure neutrals, but only the elemental ions, resulting in a higher resolution energy peak for the detected elements than DRS. In addition, MSRI instruments detect all elements with isotopic resolution, including low mass elements (i.e. molecular hydrogen and atomic deuterium) which are indistinguishable by the SIMS method. Since the recoiled MSRI ions have a much larger velocity than the SIMS ions, the MSRI ions are much less subject to neutralization by charge exchange with the surface, and, therefore, MSRI measures a large ion fraction of the ejected species, however, the number of ejected species is small.
Currently, monitoring the surface properties of thin films, especially during the growth of thin films, is critical in technologies involving diamond films, multi-component semiconductor films, and metal and metal oxide films. Thin films are grown under specific conditions, including a low vacuum, high pressure environment. For example, typical conditions for diamond growth include a hydrogen atmosphere, heating, and the allowance for the positioning of film deposition and other instruments. Key factors influencing the surface properties of thin films are the deposition rates of various species, migration of materials at the surface, differences between surface and sub-surface composition, thickness and uniformity of the film, and nucleation of growth sites. For multi-component films, and particularly for multi-component films grown in an atmosphere of oxygen or nitrogen, precise control of the film properties depends on the ability to monitor the growth process as it occurs.
Mass spectroscopy techniques employing low energy pulsed ion beams (less than or equal to 10 keV) are capable of providing a wide range of information directly relevant to the growth of thin films. However, ion beam methods have not been widely used for monitoring thin film growth, because the existing commercial designs and instrumentation are largely unsuitable for the application. For example, in order to characterize the process occurring at the surface of a growing film, the instrument must probe the first few atomic layers and identify the uppermost monolayer where the growth occurs. Most surface analysis methods, however, are unsuitable as in-situ monitors of thin film deposition processes because they require ultra-high vacuum environments, physically obstruct the deposition process, take too long to acquire data, and/or cause significant damage to the film.
One approach for adapting DRS/MSRI instruments to thin film growth applications has been to equip the ion sources and detectors with differential pumping apertures which terminate close to the sample surface, such that the high pressure path traveled by the beam is small. The high velocity of the recoiled MSRI elemental ions allows for surface analysis under high pressure conditions, if both the primary ion source and the detector(s) are differentially pumped. The ability to measure the surface composition with isotopic resolution at high sample pressures makes MSRI suitable for in-situ, real-time monitoring and process control of a variety of thin film deposition processes. SIMS analysis at high pressures, however, is not feasible due to the low velocity of the SIMS ions.
SIMS instruments and MSRI instruments provide complimentary information regarding the chemical composition and structure of the surface of a sample. SIMS provides information about the molecular and elemental species present on the surface of the sample, however, with some complexity regarding the analysis. MSRI provides more quantitative information about elemental species only, and, when used in conjunction with SIMS, can simplify the SIMS analysis. Although there are numerous ToF SIMS instruments utilizing reflectron analyzers, such instruments are not capable of MSRI analysis because MSRI ions have significantly greater energy than SIMS ions and available SIMS ToF instruments are not capable of operating at the high voltages needed for MSRI analysis. Also, the detection of MSRI ions requires an experimental geometry that is different than the geometry used in SIMS ToF measurements.
A need exists in the art for an instrument capable of performing both SIMS and MSRI measurements in a thin film growth environment. The instrument must provide a diverse range of information (composition, structure, growth), be compatible with process conditions (temperature, pressure), be non-destructive to the sample surface, operate in real time, and not interfere with the surface deposition instruments.
The present invention is a ToF SIMS/MSRI reflectron mass analyzer and method that is capable of providing mass spectrum of isotopic resolution for all elements, including hydrogen and helium, using the techniques of both SIMS and MSRI. The use of a single mass analyzer to selectively obtain pure SIMS and/or MSRI spectra is unique and provides valuable, complimentary surface information for sample materials, including thin films.
Therefore, in view of the above, a basic object of the present invention is to provide a ToF SIMS/MSRI reflectron mass analyzer and method capable of performing surface analysis on thin films using both SIMS and MSRI techniques. In addition, MSRI analysis may be performed during thin film growth, in a low vacuum, high pressure environment.
A further object of this invention is to provide a ToF SIMS/MSRI reflectron mass analyzer and method of using a reflectron time of flight analyzer having a critical, optimal geometry, and adjustable reflectron voltages and extraction optics, such that SIMS measurements and MSRI measurements may be accomplished with the same instrument.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.